Suspected militants attacked and set fire to at least 20 oil tankers in Pakistan en route to Nato forces in Afghanistan, police said tonight.

Three people were killed in the attack, which occurred close to Islamabad, the capital, according to senior police official Umar Hayat.

It was the third time since Friday that Nato convoys travelling by land through Pakistan to Afghanistan have been targeted.

"Gunmen opened fire and then set on fire tankers parked on the roadside. It's a big fire. We're trying to control it," Hayat said.

He said the attack had been carried out by "terrorists". The trucks were parked at a poorly guarded terminal when they were set alight.

The tankers were travelling to the Torkham border crossing at one end of the Khyber Pass, one of the main routes used to carry fuel, military vehicles, spare parts, clothing and other non-lethal supplies to foreign troops in Afghanistan. The crossing has been closed for days after three Pakistani troops were killed last week in a helicopter strike near the border by the military alliance in a border area.

The latest attacks are likely to delay the reopening of the supply route, which has heightened tensions between the US and Pakistan.

Pakistan's other main route into Afghanistan, in Chaman in the southwest, remains open.

Nato and the US have alternative supply routes into Afghanistan, but the Pakistani ones are the cheapest and easiest. Most of the coalition's non-lethal supplies are transported over Pakistani soil after they are unloaded at docks in Karachi.

Over the past two years, the convoys, which take several days to reach the border after leaving the port city, have been targeted by militants mostly in the northwestern border area.They have also been attacked by criminals wanting to sell the clothing, vehicles and other equipment they carry.